xen: drop xen_sched_clock in favour of using plain wallclock time

xen: drop xen_sched_clock in favour of using plain wallclock time

commit 8a22b9996b001c88f2bfb54c6de6a05fc39e177a upstream.

xen_sched_clock only counts unstolen time.  In principle this should
be useful to the Linux scheduler so that it knows how much time a process
actually consumed.  But in practice this doesn't work very well as the
scheduler expects the sched_clock time to be synchronized between
cpus.  It also uses sched_clock to measure the time a task spends
sleeping, in which case "unstolen time" isn't meaningful.

So just use plain xen_clocksource_read to return wallclock nanoseconds
for sched_clock.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>